Field of Invention
The present invention relates to an electronic field, and more particularly to a power-adjustable RF (radio frequency) output circuit.
Description of Related Arts
At present, in RF power fields, radio and television fields, etc., there are two methods to conveniently adjust the RF output power with the needs of users. The first method is to adjust the RF input signal amplitude. The second method is to adjust the main power supply voltage on the drain of the power amplifier module.
The first method adopts DDS (Direct Digital Synthesizer) to generate the RF frequency source, which is more adapted for the broadband RF power supply or transmitter. It is unable to adopt the crystal with temperature compensation to simply generate a fixed and precise RF frequency source, so that it becomes complex and much higher in control cost.
In the second method, the RF frequency source is simply generated by the crystal with temperature compensation, but the main voltage supplied to the drain of the power amplifier module must be variable. However, the switching power supply voltage is made into a fixed output, which is better controlled, easier and more stable than a variable output.
The MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) is a voltage-controlled component, that is, the working state of the MOSFET varies with changing the control voltage applied to the gate. The deep negative bias voltage allows the MOSFET to stop working. In the above-mentioned two methods for changing the RF output power, the bias voltage at the gate of the power amplifier module is set to a fixed value (such as 2.5 VDC, 2.8 VDC, or 3 VDC).